diff --git a/src/relay/dispatcher-notification-ownership.test.ts b/src/relay/dispatcher-notification-ownership.test.ts
new file mode 100644
index 00000000000..e94fdcede62
--- /dev/null
+++ b/src/relay/dispatcher-notification-ownership.test.ts
@@ -0,0 +1,18 @@
+import { describe, expect, it, vi } from 'vitest'
+import { RelayDispatcher } from './dispatcher'
+
+describe('RelayDispatcher notification ownership', () => {
+  it('refuses a second handler for a method instead of silently shadowing the first', () => {
+    const dispatcher = new RelayDispatcher(() => {})
+    const owner = vi.fn()
+
+    dispatcher.onNotification('pty.ackData', owner)
+
+    // Why: the slot holds one handler, so a second registration used to win purely by
+    // construction order — that is how a no-op ack handler nearly disabled credit acks.
+    expect(() => dispatcher.onNotification('pty.ackData', vi.fn())).toThrow(/already registered/)
+
+    // Why: the constructor arms a keepalive interval that would outlive the test.
+    dispatcher.dispose()
+  })
+})
diff --git a/src/relay/dispatcher.ts b/src/relay/dispatcher.ts
index 1898e9fbf4e..00e1836d9d7 100644
--- a/src/relay/dispatcher.ts
+++ b/src/relay/dispatcher.ts
@@ -212,7 +212,14 @@ export class RelayDispatcher {
     this.requestHandlers.set(method, handler)
   }
 
+  // Why it throws: this is a single slot, so a second registration silently shadows the
+  // first and which one survives depends only on construction order. `pty.ackData` shipped
+  // that way — a no-op handler was saved from disabling credit acks purely by the adapter
+  // being constructed second (STA-4571). Fail loudly instead of encoding that ordering.
   onNotification(method: string, handler: NotificationHandler): void {
+    if (this.notificationHandlers.has(method)) {
+      throw new Error(`Notification handler for ${method} is already registered`)
+    }
     this.notificationHandlers.set(method, handler)
   }
 
diff --git a/src/relay/pty-handler-spawn-admission.test.ts b/src/relay/pty-handler-spawn-admission.test.ts
index c5938d91f57..aae35a2e166 100644
--- a/src/relay/pty-handler-spawn-admission.test.ts
+++ b/src/relay/pty-handler-spawn-admission.test.ts
@@ -79,7 +79,10 @@ describe('PtyHandler', () => {
     const notifMethods = Array.from(dispatcher._notificationHandlers.keys())
     expect(notifMethods).toContain('pty.data')
     expect(notifMethods).toContain('pty.resize')
-    expect(notifMethods).toContain('pty.ackData')
+    // Why not `toContain('pty.ackData')`: PtyHandler must NOT own that method. It used to
+    // register a no-op here, which survived only because the consumer session adapter was
+    // constructed later and overwrote it (STA-4571).
+    expect(notifMethods).not.toContain('pty.ackData')
   })
 
   it('rejects strict process inspection for a missing relay PTY', async () => {
diff --git a/src/relay/pty-handler.ts b/src/relay/pty-handler.ts
index c7dfb0a533d..3f82038261d 100644
--- a/src/relay/pty-handler.ts
+++ b/src/relay/pty-handler.ts
@@ -903,9 +903,6 @@ export class PtyHandler {
 
     this.dispatcher.onNotification('pty.data', (p) => this.writeData(p))
     this.dispatcher.onNotification('pty.resize', (p) => this.resize(p))
-    this.dispatcher.onNotification('pty.ackData', (_p) => {
-      /* flow control ack -- not yet enforced */
-    })
   }
 
   private isLikelyInteractiveRedraw(data: string): boolean {